Before upgrading to High Sierra, I wanted to determine which of my antiquated apps and drivers would get broken in the process. I cloned my boot drive (an SSD) to an external USB 2.0 HDD (no USB 3.0 on a mid-2010 iMac), booted into the clone, and then proceeded to install High Sierra.
That went as expected, and disk speeds were acceptable given the limitations of USB 2.0. However, to get a full sense of what to expect compatibility wise I needed to convert the drive to AFPS, and preferably enable FileVault.
I booted into recovery mode and converted the volume to AFPS, resulting in a non-bootable drive. I considered the possibility that APFS simply did not play well with this particular drive, so I formatted the volume as APFS in recovery mode and attempted a fresh (non-cloned) installation. It went as expected.
With that question out of the way, I went back to trying to convert an HFS+ clone of my boot drive to AFPS. I took a different approach this time, wherein I used a command line toggle to force High Sierra to do conversion during the installation. The install seemed to go well, but got stuck and made no progress for a couple hours (in fact, the drive actually spun down).
I had tried different variations of the steps above but nothing worked — except this:
1) Erased the USB drive and split it into two HFS+ partitions.
2) Cloned my Sierra install to the first partition on the USB HDD.
3] Updated Sierra on the clone drive to High Sierra.
4) Formatted the second partition as APFS.
5) Cloned High Sierra install to AFPS partition.
As others have noted on the forums, APFS performance is HORRIBLE on external drives. One person claimed that boot times were twice as long when compared to HFS+; I would argue that they are three times that. The responsiveness of the APFS (HDD) partition can be likened to booting from a Linux live CD on a 4x optical drive, but somehow worse.
That went as expected, and disk speeds were acceptable given the limitations of USB 2.0. However, to get a full sense of what to expect compatibility wise I needed to convert the drive to AFPS, and preferably enable FileVault.
I booted into recovery mode and converted the volume to AFPS, resulting in a non-bootable drive. I considered the possibility that APFS simply did not play well with this particular drive, so I formatted the volume as APFS in recovery mode and attempted a fresh (non-cloned) installation. It went as expected.
With that question out of the way, I went back to trying to convert an HFS+ clone of my boot drive to AFPS. I took a different approach this time, wherein I used a command line toggle to force High Sierra to do conversion during the installation. The install seemed to go well, but got stuck and made no progress for a couple hours (in fact, the drive actually spun down).
I had tried different variations of the steps above but nothing worked — except this:
1) Erased the USB drive and split it into two HFS+ partitions.
2) Cloned my Sierra install to the first partition on the USB HDD.
3] Updated Sierra on the clone drive to High Sierra.
4) Formatted the second partition as APFS.
5) Cloned High Sierra install to AFPS partition.
As others have noted on the forums, APFS performance is HORRIBLE on external drives. One person claimed that boot times were twice as long when compared to HFS+; I would argue that they are three times that. The responsiveness of the APFS (HDD) partition can be likened to booting from a Linux live CD on a 4x optical drive, but somehow worse.